In recent years, it has been strongly desired in the coating art, particularly the automobile coating art that coating process should be simplified and reduced so as to solve the problems of resource saving, cost saving, environmental load (such as VOC and HAPs) reducing, and the like.
In a conventional process for forming a layered coated film, each of coating compositions used to form a layered coated film, has been separately applied and cured. As to an automobile coating process for example, after an electrodeposition coating composition has been applied and cured, curing step is conducted 3 times in every coating step for an intermediate coating composition, an base coating composition, and a clear coating composition. This conventional process for automobile coating is generally referred to as 3 coat 3 bake coating process.
In order to simplify the automobile coating process, 3 coat 1 bake coating process has also been proposed. In this coating process, an intermediate coating composition, an base coating composition, and an top coating composition are sequentially applied on the cured electrodeposition coated film by wet on wet, thereafter the layered wet coated film is heated and cured together. Thereby, curing process is reduced to one time.
However, if the 3 coat 1 bake coating process is conducted with using a conventional electrodeposition coating composition, it is insufficient in throwing power, unevenness of film thickness occurs, and the thicker part leads wasteful consumption of the coating composition. Further, if the electrodeposition coated film is uneven in thickness, the coated films applied thereon becomes poor in surface smoothness.
In addition, if the 3 coat 1 bake coating process is conducted with using a conventional intermediate coating composition and base coating composition, coated layers may be solubilized with each other or inverted, and appearance of the coated surface becomes poor.